Worldwide over 10 million individuals have become infected with the human immunodeficiency virus (HIV) and over three quarters of these infections have been heterosexually acquired. Despite behavioral change, HIV seroprevalence rates among heterosexual populations continue to rise, particularly in subsaharan African and southeast Asia. The objective of this proposal is to develop cohorts of individuals at high risk for HIV infection in preparation for HIV vaccine efficacy trials. A cohort of prostitutes and a cohort of truck drivers will be recruited in Mombasa, Kenya and followed at the municipal sexually transmitted disease (STD) clinic. The specific aims of this prospective study are to 1) determine-HIV seroincidence in each cohort, 2) identify correlates of HIV seroconversion, including STDS, circumcision status, cervical ectopy, and sexual practices, 3) determine the effect of circumcision status and cervical ectopy on the incidence of STDS, and 4) evaluate the temporal sequence of HIV strain variation by characterizing viral phenotype and genotype at the time of seroconversion and at six months of followup. Registered prostitutes who attend the Mombasa STD Clinic and long distance truck drivers employed by two large trucking companies in Mombasa will be screened for the presence of HIV antibodies and invited to participate in a prospective cohort study if HIV seronegative. An interview, physical examination, and evaluation for STDs will be conducted at enrollment and at followup visits. Prostitutes will be followed at monthly intervals and truck drivers at three monthly intervals. The presence of STDs among women will be directly assessed by clinical examination, vaginal swab microscopy, and routine microbiology. The incidence of new genital infections among truck drivers will be assessed through serologic evaluation for C. trachomatis, H. ducreyi, Herpes simplex virus type 2, and T. pallidum. HIV serologic testing will be performed at each follow-up visit and peripheral blood lymphocytes will be collected from individuals who seroconvert for viral strain analysis. Kaplan Meier survival analysis and Cox proportional hazards models will be used to determine HIV seroincidence and correlates for seroconversion in each of the cohorts. These studies will provide data regarding factors which influence HIV transmission efficacy which may lead to new prevention interventions. In addition, it is anticipated that these cohorts will form the basis for future HIV vaccine trials in Kenya.